The present invention relates to a method for controlling exposure light beams for use in a picture reproducing machine such as a color scanner and a color facsimile.
A color scanner for plate making, which is capable of reproducing a halftone picture image directly without using a halftone screen, has heretofore been developed. In a picture recording unit of such a color scanner, in order to promote the processing speed without impairing the resolving power of the reproduction picture, usually, the scanning speed in the direction of a cylinder's periphery is increased, or optical recording scanning number is increased to several with respect to one mechanical scanning line and the scanning speed in the direction of a cylinder's axis is raised.
However, in the former method, the scanning speed in the direction of the cylinder's periphery of the color scanner having cylinders is restricted, and this scanning speed now practiced is near a mechanically critical speed. Hence, the scanning speed in the direction of the cylinder's axis is to be increased.
Then, in order to use a plurality of scanning lines in a single operation, an exposure means including control means which can generate and control a plurality of exposure light beams in parallel corresponding to such scanning lines, are required.
In the prior art, each conventional exposure light beam control means for performing such a method comprises an electronic shutter means composed of a combination of an electrooptical deflector element and a deflector filter, as disclosed in Japanese Patent Publication No. 52-33523.
In this case, however, a highly accurate adjustment of each light axis is required. Accordingly, the arrangement of a plurality of exposure light beam control means involves the complicated adjustments of the light axes of the exposure means, with the result of the lowering of the long-range stability. Thus, it requires a readjustment of them after a certain period of use, and the readjustment involves plenty of time and troubles.
Further, in such an exposure means, the exposure light beams are usually produced by dividing the light beam generated by a laser by using a plurality of half mirrors, and hence the luminous energy of the light beams divided are lowered.
Another method for obtaining a plurality of exposure light beams by impressing a plurality of supersonic waves of different frequencies to an acoustooptical deflector element has been developed, as disclosed in Japanese Patent Laying-Open Specification No. 51-90601. In this case, since the supersonic waves having the different frequencies are supplied to the acoustooptical deflector element, in order to remove the influence of the interferences of their frequencies mono-spectral supersonic waves should be used accordingly.
Furthermore, since the acoustooptical deflector element allows to receive a restricted power of a laser beam, when the number of the exposure light beams is increased, the luminous energy of each exposure light beam equally divided may be largely reduced, and the deflection angles of first-order diffraction light beams corresponding to the frequencies of the supersonic waves are close to one another. Accordingly, an arrangement and a position adjustment of optical systems for separating the light paths of the exposure light beams became complicated, and further stability of the frequencies of the supersonic waves should be raised.